A flash memory is a non-volatile storage device that can be electrically erased and reprogrammed. Flash memories are commonly used in memory cards, USB flash drives and solid-state drives for general storage and transfer of data between computers and other digital products. Flash memory devices typically store information in an array of memory cells made using floating gate transistors.
A floating gate transistor is a field effect transistor having a structure similar to a conventional MOSFET (metal oxide semiconductor field effect transistor). Floating gate MOSFETs are distinguished from conventional MOSFETs because the floating gate transistor includes two gates instead of one. In addition to an upper control gate, a floating gate transistor includes an additional floating gate beneath the control gate and above the transistor channel but completely electrically isolated by an insulating layer such as an oxide layer that completely surrounds the floating gate. This electrically isolated floating gate creates a floating node in DC with a number of inputs or secondary gates such as the control gate, formed above the floating gate and electrically isolated from it. These secondary gates or inputs are only capacitively connected to the floating gate. Because the floating gate is completely surrounded by highly resistive material, i.e. the insulating layer, any charge placed on the floating gate is trapped there and the floating gate remains unchanged for long periods of time until the floating gate MOSFET is erased. Unless erased, the floating gate will not discharge for many years under normal conditions. Fowler-Nordheim Tunneling or other Hot-Carrier injection mechanisms may be used to modify the amount of charge stored in the floating gate, e.g. to erase the floating gate. The erase operation is therefore critical to the operation of floating gate transistors.
The default state of an NOR (“Not Or” electronic logic gate) flash cell is logically equivalent to a binary “one” value because current flows through the channel under application of an appropriate voltage to the control gate when charge is stored in the floating gate. Such a flash cell device can be programmed or set to binary “zero” by applying an elevated voltage to the control gate.
To erase such a flash cell, i.e. resetting it to the “one” state, a large voltage of the opposite polarity is applied between the control gate and the source causing electrons to exit the floating gate through quantum tunneling. In this manner, the electrical charge is removed from the floating gate. This tunneling necessarily takes place through the inter-gate dielectric formed between the floating gate and the control gate. The inter-gate dielectric extends over the edge of the floating gate and also over a floating gate oxide formed over the floating gate and the tunneling typically takes place through the inter-gate dielectric at the edge of the floating gate. The configuration and shape of the floating gate and the inter-gate dielectric have a significant impact on tunneling and the ability to erase the flash cell. It is therefore important to provide a floating gate transistor having a floating gate electrode, floating gate oxide and inter-gate dielectric appropriately configured to promote the creation of a strong electric field that enables tunneling and allows for the flash cell device to be easily erased.
The present invention addresses these concerns.